1. Field of the Invention
The present invention relates to a circuit and method for coding input signals from an audio output device, particularly to a circuit and method for coding input signals to a 3-wired interface for a pair of stereo differential pulse coded headphones.
2. Description of the Prior Art
FIG. 1 is a diagram showing a pair of conventional analog 3-wired headphones 11, with a right and left speakers 121, 122. Each of the speakers 121, 122 gives off sound by a differential voltage signal from the terminal A and B, or C and D. The headphones 11 have three input terminals 111, 112 and 113 for receiving the external analog voice signal. Both terminals B and D are connected to the same input terminal 112 to share a common ground reference voltage, while the terminals A and C are connected to the input terminals 111 and 113 respectively. Wherein the terminal 111 receive an analog voice signal VSR from a right channel and the terminal 113 receive an analog voice signal VSL on from left channel for generating the stereo sound.
However, the conventional analog 3-wired headphones demand high power consumption due to the analog input voice signals.
FIG. 2 is a diagram showing a 4-wired headphone 21 with a right and left speaker 221 and 222. Each of the speakers 221 and 222 gives off sound by a differential voltage signal from the terminal A and B, or C and D. The headphone 21 has four input terminals 211, 212, 213 and 214. The terminals B and D are connected to the input terminal 212 and 213 respectively to receive their corresponding reference voltages respectively, while the terminals A and C are connected to the input terminal 211 and 214 respectively to receive an analog voice signal VSR on a right channel and an analog voice signal VSL on a left channel for generating the stereo sound. There are two differential input pairs in the 4-wired headphones.
However, the 4-wired headphones are not compatible with most existing audio headphone systems with 3-wired plugs.